1. Field
The present invention relates to a delay filter. More specifically, the present invention relates to a bipolar optical delay line filter.
2. Description of Related Art
Fiber optic delay line filters are devices that can be used for RF or microwave signal processing. The filters can be configured to perform bandpass, band-stop, apodizing, comb-selection and a variety of other filtering functions heretofore performed by conventional microwave filters.
Fiber optic delay line filters have many properties that make them attractive for high-frequency applications. The optical fibers that are used to generate the delay have negligible dispersion, almost no loss, and occupy a relatively small volume, even when the individual fibers are long. By comparison, microwave lumped-element and tapped delay line filters are lossy, have considerable dispersion, and can be quite bulky and expensive to build at the higher microwave frequencies where waveguides must be used.
A typical fiber optic delay line filter consists of a length of optical fiber, with equally-spaced signal taps distributed along its length. The signal taps are used to remove a percentage of light that propagates through the filter. The small amount of light that is removed at each tap is combined with the light extracted from other signal taps and fed into a photodetector. This photodetector converts the light into an electrical current.
The intensity of the light entering the fiber optic delay line filter is amplitude-modulated by a microwave signal. Thus, the resultant electrical current at the photodetector will be equivalent to the amplitude of the sum of a number of microwave signals, each delayed by an amount nτ, where n is the tap number and τ is the delay time (the time of propagation) between taps. If the tap strengths and delay lengths are chosen correctly, one can realize a number of different filter configurations.
The filter versatility, however, is limited by the fact that the current generated at the photodetector will always flow in one direction. Using this configuration, one cannot generate bipolar (plus and minus) signals. This is a severe restriction that limits the type of filter that can be built to a filter having a bandpass maximum at zero frequency.
To overcome this, the bipolar optical delay line filter 10 shown in FIG. 1 has been used. Such a bipolar filter is discussed in Jose Capman, Joaquin Cascon, Jose Luis Martin, Salvador Sales, Daniel Pastor, and Javier Marti, “Synthesis of Fiber-Optic Delay Line Filters,” Journal of Lightwave Technology, Vol. 13, pp. 2003-2012 (1995).
The filter 10 consists of an optical splitter 12 for receiving and splitting an optical signal, a plurality of tap elements 14 to extract light from the optical signal, a summer 18 for combining the extracted light, a photodetector, comprised of a first and second photodiode 20, 22, and an amplifier 24.
The optical signal enters the optical splitter 12 where the optical signal is split into halves, thereby generating a first optical signal and second optical signal. The first optical signal is coupled into a first delay line 13 and proceeds to the upper set 3 of tap elements 14 and optical delay loops 16. The second optical signal is coupled into a second delay line 15 and proceeds to the lower set 5 of tap elements 14 and optical delay loops 16. The first and second optical signals proceed through the tap elements 14, which extract a percentage of light from the first and second optical signals. The light extracted by each tap element 14 from the first optical signal is received by the summer 18 and used to illuminate the first photodiode 20. The light extracted by each tap element 14 from the second optical signal is received by the summer 19 and used to illuminate the second photodiode 22. The first and second photodiodes 20, 22 convert the extracted light into an electrical current.
The electrical current from the first and second photodiodes 20, 22 is coupled to an amplifier 24 to generate an electrical signal that is proportional to the algebraic difference of the first and second optical signals received by summers 18, 19. When the amplifier 24 is connected to an oscilloscope or a similar device, the electrical current can be viewed. Because the first and second photodiodes 20, 22 are used in a push-pull configuration, current can be either injected into or drawn from the amplifier 24. The electrical current that results from the tap elements 14 in the lower set 5 will appear as a negative signal while the electrical energy that results from the tap elements 14 in the upper set 3 will appear as a positive signal. Thus, the first and second optical signals, though both initially positive, generate positive and negative electrical signals as they proceed through the filter 10.
The filter 10 shown in FIG. 1 also contains several optical delay loops 16 consisting of optical fiber. As the first and second optical signals proceed through the tap elements 14, they travel through the optical loops 16. The optical delay loops 16 delay the time it takes for the first and second optical signals to travel between the tap elements 14. The delay time between tap elements 14 in the upper set 3 sets the spacing between the positive signals, while the delay time between elements 14 in the lower set 5 sets the spacing between the negative signals. In this way, adjusting the amount of fiber in the optical delay loops 16 can be used to adjust the spacing between the positive or negative signals.
Additionally, the second delay line 15 contains an extra optical delay loop 17 adjacent to the optical splitter 12. The purpose of this optical loop 17 is to prevent the positive and negative signals from occurring at the same time in the time domain.
The problem with this type of bipolar optical delay line filter 10 is that numerous tap elements 14 and optical delay loops 16 are needed to extract light and to set the delay time between subsequent positive and negative signals. Generally speaking, the more complex the desired response from the extracted light, the greater the number of tap elements 14 needed to extract light from the optical signal. Furthermore, a perfect replication of any filter requires, in theory, an infinite number of tap elements to extract light. As shown in FIG. 1, there are five tap elements 14 in the upper set 3 and lower set 5. This would result in five positive signals, and five negative signals. A filter with only five tap elements 14 in each of the upper set 3 and lower set 5 would not handle a complex response as well as a filter having an infinite number of tap elements. However, increasing the number of tap elements 14 in the filter 10 would increase the size, weight, complexity, and cost of the filter.
For a certain limited class of filters, this problem can be addressed by using a recursive delay line. A recursive delay line is a loop of fiber with a single tap element. An optical signal is injected into the recursive delay line and passes through a tap element. Each time the optical signal passes through the tap element, a small percentage of light is extracted and coupled to a photodetector. The remaining light stays in the loop and makes another trip, after which the tap element extracts more light. This process is repeated, ad infinitum, until the light remaining in the fiber asymptotically approaches zero. However, this recursive delay line can only produce unipolar pulses. To get a bipolar response, one would need two recursive loops, which increases the size, cost, and complexity of the filter. Such a technique is also discussed in Jose Capman, Joaquin Cascon, Jose Luis Martin, Salvador Sales, Daniel Pastor, and Javier Marti, “Synthesis of Fiber-Optic Delay Line Filters,” Journal of Lightwave Technology, Vol. 13, pp. 2003-2012 (1995).